Infusion pumps are well known devices used to administer drugs to a patient in small, metered doses at frequent intervals or, alternatively, in the case of some devices, at a low but essentially continuous rate. Infusion pump therapy may be electronically controlled to deliver precise, metered doses at exactly determined intervals, thereby providing a beneficial gradual infusion of medication to the patient. In this manner, the infusion pump is able to mimic the natural process whereby chemical balances are maintained precisely by operating on a continuous time basis.
One of the essential elements of an infusion pump is a one-way valve, also known as a check valve or anti-free-flow valve, one or more of which may be required in virtually any design for an infusion pump. Such a valve must be highly precise, operating in a passive manner to open with a relatively small break pressure or cracking pressure in the desired direction of flow through the valve. The valve must also be resistant to a substantially higher reverse pressure, not opening or leaking at all, since any reverse flow in the opposite direction would result a reduction in the amount of medication being delivered, and an imprecise infusion pump which would be totally unacceptable.
U.S. Pat. No. 2,462,189 to Hess, U.S. Pat. No. 2,497,906 to Peters et al., U.S. Pat. No. 4,141,379 to Manske, and U.S. Pat. No. 4,593,720 to Bergandy describe check valves.
One of the better known check valves of the art is described in U.S. Pat. No. 4,712,583 to Pelmulder et al., the disclosure of which is incorporated herein by reference. The valve of Pelmulder et al. is molded in a unitary fashion of a medical grade elastomer such as silicone rubber. A circular valve disk has on the top side thereof a protruding cylindrical dynamic sealing ridge, which is the actual valve element. A static seal ring having a larger inner diameter than the outer diameter of the valve disc is located concentrically around the valve disk. The valve disk is supported from the static seal ring by a thin support web extending between the static support ring and the valve disk, which web has a plurality of holes therethrough to allow fluid passage.
The valve is installed by locating it in a first housing portion which has provision, such as an annular groove, for receiving the static seal ring, and which also includes a web support structure for supporting a portion of the web adjacent to the static seal ring. The first housing portion has an aperture therein to allow fluid passing through the valve to exit, which aperture is located on the underside of the valve when it is installed in the first housing portion as described above.
A lower housing portion is then installed on top of the valve as previously installed in the first housing portion. The second housing portion, which rests on top of the valve, is essentially flat, and has an aperture therein through which fluid may enter towards the valve. This aperture is located above the valve disk and concentrically within the dynamic sealing ridge. When the lower housing portion is installed onto the first housing portion with the valve therebetween, the static seal ring is compressed to create a good seal.
In operation, when the pressure is greater on top of the valve disk than under the valve disk, the valve will tend to open, requiring only a small pressure to operate. However, when this small break pressure is not present, or when a reverse pressure is present, the valve will remain in a closed position. The valve thus has a positive sealing action when closed, and opens easily when the small crack pressure (or a greater pressure in that direction) is present.
It is essential in the teachings of Pelmulder et al. that the portion of the housing mounted on the top side of the valve be flat and that the dynamic sealing ridge be bulgy so as to space the web from the flat surface of the top housing.
U.S. Pat. No. 5,025,829 to Edwards et al., the disclosure of which is incorporated herein by reference, describes a check valve disk which comprises a flat, circular piece of flexible material. The valve disk is formed by an outer sealing ring at its periphery, at least three arcuately-shaped windows substantially evenly-spaced radially inwards from the outer sealing ring, and a circular closing member concentrically disposed radially inwards from the windows and joined to the outer sealing ring by a plurality of webs. The valve disk is installed between a first housing portion having an inlet and a valve seat, and a lower housing portion having an outlet. Both housing portions have a valve disk retaining surface with an annular ridge protruding therefrom. In such a manner, the valve disk is retained between the housing portions by the annular ridges contacting its outer sealing ring.
The valve seat which is formed in the first housing portion protrudes towards the lower housing portion by a preselected distance beneath the valve disk retaining surface in the first housing portion. In such a manner, various amounts of preloading are capable with valve disks of the same manufacture. Edwards et al. teaches constructing the valve disk as a flat, circular piece of flexible material. The housing portions are snap-fit together.
The present inventors have found that both the Pelmulder et al. check valve and the Edwards et al. check valve suffer from drawbacks. Because the Edwards et al. check valve employs a flat valve disk, it has been difficult to control accuracy of the cracking pressure. The dynamic sealing ridge of the Pelmulder et al. check valve does not satisfactorily provide accurate control of the cracking pressure either, possibly due to its elastomeric properties.